Michael Flynn, President Donald Trump’s former national security adviser, has rejected a new request to appear before the Senate Intelligence Committee, CNN reported on Tuesday night.

It was not immediately clear what prompted the latest request from the congressional panel, which previously subpoenaed Flynn in May for documents related to his communications with Russian operatives between 2015 and 2017 as part of its investigation into the Trump campaign’s ties to Russia.

His attorney, Robert Kelner, said at the time that Flynn was pleading the fifth because of the “escalating public frenzy against him.”Flynn has become a central figure in the Russia investigation because of his history of contact and engagement with the Kremlin before and during the 2016 election. He has sought immunity in exchange for his testimony, but neither the House nor the Senate committees have accepted that offer, CNN reported.

After Flynn’s Fifth Amendment plea earlier this year, the Senate Intelligence Committee subpoenaed two of his Virginia-based businesses instead. Committee’s chair Sen. Richard Burr and ranking member Sen. Mark Warner issued a warning to Flynn after the plea:

“While we recognize General Flynn’s constitutional right to invoke the Fifth Amendment, we are disappointed he has chosen to disregard the Committee’s subpoena request for documents relevant and necessary to our investigation. We will vigorously pursue General Flynn’s testimony and his production of any and all pertinent materials pursuant to the Committee’s authorities.”

The Senate is conducting its Russia probe alongside the House Intelligence Committee. Special counsel Robert Mueller and a team of prosecutors are also deep into their own inquiry into whether the Trump campaign colluded with Russian operatives during the 2016 election.